CARACAS, Jan 3 (Reuters) - Venezuelan President Hugo Chavezis still suffering a "severe" respiratory infection that hashindered his breathing as he struggles to recover from cancersurgery in Cuba, the government said on Thursday.

The 58-year-old socialist leader has not been seen in publicnor heard from in more than three weeks. Officials say he is indelicate condition after his fourth operation in just 18 monthsfor an undisclosed form of cancer in his pelvic area.

"Comandante Chavez has faced complications as a result of asevere lung infection," Information Minister Ernesto Villegassaid in the latest official update on the president's condition.

"This infection has caused a breathing insufficiency thatrequires Comandante Chavez to comply strictly with medicaltreatment," the communique added, giving no further details.

Vice President Nicolas Maduro had earlier returned toVenezuela on Thursday after visiting Chavez in hospital asrumors swirled that the president could be close to death.

Flanked by senior government figures including DiosdadoCabello, the head of the National Assembly, Maduro toured acoffee production plant in Caracas - the type of visit that thepresident made frequently before he fell ill.

"He is conscious of the battle that he's in, and has thesame fighting spirit as always, with the same strength andenergy as always, with his confidence and security," Madurosaid. "We're going to be alongside him with the same strengthand the same energy."

Maduro said Cabello, Oil Minister Rafael Ramirez andChavez's elder brother Adan, among others, had all been with thepresident in the Havana hospital.

Venezuelan bonds rallied to five-year highs earlier onThursday on rumors that Chavez's health had taken a turn for theworse. Foreign investors generally hope for a morebusiness-friendly government in Venezuela, and its assets haverallied in recent months on news of his illness.

In scenes that recalled Chavez's hours-long televised visitsto building sites, hospitals and oil refineries, Maduro toldworkers at the nationalized Fama de America factory that therewas no "transition" taking place in the country.

"The only transition in Venezuela is the transition tosocialism," he said in comments carried live by statetelevision.

"It began six years ago, ordered by Comandante Hugo Chavezas chief and president, elected, re-elected and ratified, muchas it pains the bourgeois hucksters and the right, who have doneso much damage to our fatherland."

Chavez's abrupt exit from the political scene would be ahuge shock for the South American OPEC nation. His oil-financedsocialism has made him a hero to the poor majority but criticscall him a dictator.

His condition is being watched closely by Latin Americanallies that have benefited from his help, as well as investorsattracted by Venezuela's lucrative and widely traded debt.

'MAKE NO MISTAKE'

Chavez is still set to be sworn in on Jan. 10, as spelledout in the constitution. If he were to die or had to step aside,new elections would be held within 30 days, with Maduro runningas the ruling Socialist Party (PSUV) candidate.

While the constitution gives Jan. 10 as the start of a newpresidential term, it does not explicitly state what happens ifa president-elect cannot take office on that date.

Top PSUV officials have suggested that Chavez's inaugurationcould be postponed - while the opposition says any delay wouldbe just the latest sign the former soldier is not fit to govern.

Cabello said the "Chavismo" movement was in pain butremained resolute, and he issued a warning to the opposition:"Make no mistake about these people or this revolution. It isgoing to cost you very, but very, dearly," he said.

On Saturday, Cabello will likely be re-elected as head ofthe Chavista-dominated National Assembly, a key post that couldsee him assume Chavez's role temporarily while new elections arecalled should the president have to step down.

In the past Cabello has been considered as a rival ofMaduro, but the pair have been at pains to deny that. Theirappearance side-by-side at the coffee factory on Thursday lookedto be the latest effort to project a unified front.

Last year, Chavez staged what appeared to be remarkablecomeback from the disease to win re-election to a new six-yearterm in October despite being weakened by radiation therapy. Buthe returned to Cuba for more treatment within weeks of his win.

Officials have said he suffered unexpected bleeding and thena respiratory infection after a six-hour operation on Dec. 11.That respiratory infection caused further complications, theyhave said, without giving more details.

The head of the opposition's Democratic Unity coalition,Ramon Aveledo, has accused the authorities of breaking a pledgeto keep Venezuelans informed about Chavez's health.

And one opposition leader suggested on Thursday thatlegislators should form an official commission to visit Cuba andassess the president's condition for themselves.

Maduro hit back in his televised comments, saying the publichad been provided with updates almost every day, and he accusedAveledo of orchestrating a campaign of misinformation.

"We have no doubt Mr. Aveledo is behind the campaign of sickrumors that began on Twitter and Facebook," Maduro said.